In accordance with the recent increasing storage capacity in information processing, various types of information recording technologies have been developed. In particular, the surface recording density of an HDD (hard disk drive) using a magnetic recording technology has been increasing at an annual rate of about 100%. Recently, 2.5-inch-diameter magnetic disks used in HDDs and the like also have been required to have an information recording capacity of larger than 100 GB per disk. In order to satisfy such a requirement, it is necessary to achieve an information recording density of higher than 150 Gbits per square inch.
In order to achieve a high recording density in a magnetic disk used in an HDD or the like, it has been necessary to reduce the size of crystalline magnetic particles constituting a magnetic recording layer for recording information signals and simultaneously to decrease the thickness of the layer. However, in a case of a magnetic disk of a conventionally commercialized in-plane magnetic recording system (also called a longitudinal magnetic recording system or a horizontal magnetic recording system), as a result of the progress in the size reduction of crystalline magnetic particles, thermal stability of recorded signals is deteriorated by the superparamagnetic phenomenon. This causes a so-called thermal fluctuation phenomenon in which the recorded signals are erased. Thus, the reduction in size of crystalline magnetic particles has been a factor that inhibits an increase in recording density of the magnetic disk. In order to solve the inhibitory factor problem, recently, a magnetic disk of a perpendicular magnetic recording system has been proposed.
In the perpendicular magnetic recording system, the axis of easy magnetization of a magnetic recording layer is adjusted so as to be oriented in the direction perpendicular to a surface of a substrate, unlike the case of the in-plane magnetic recording system. The perpendicular magnetic recording system can suppress the thermal fluctuation phenomenon compared to the in-plane recording system and is therefore suitable for increasing the recoding density.
In the perpendicular magnetic recording system, when the magnetic recording layer has an hcp structure (hexagonal close-packed structure), the axis of easy magnetization is the C-axis direction, and the C-axis is required to be oriented in the normal direction of a substrate. In order to enhance the orientation of the C-axis, as shown in Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2003-77122), it is effective to provide a non-magnetic under layer of an hcp structure below the magnetic recording layer.
Furthermore, in the perpendicular magnetic recording system, the S/N ratio (signal/noise ratio) and the coercivity Hc can be improved by forming the magnetic recording layer so as to have a granular structure in which grain boundaries are formed by segregating a non-magnetic material (mainly an oxide) among magnetic particles (magnetic grains) for isolating the magnetic particles and reducing the size of the magnetic particles. Patent Document 2 (Japanese Unexamined Patent Application Publication No. 2003-217107) discloses a constitution in which a columnar granular structure is formed by allowing epitaxial growth of the magnetic particles.
Furthermore, in the perpendicular magnetic recording system, as described in Patent Document 3 (Japanese Unexamined Patent Application Publication No. 2002-74648), it is preferred to use a so-called two-layer type perpendicular magnetism in which a soft magnetic layer is provided under the magnetic recording layer. With this, the magnetic field intensity (flux density) can be increased by concentrating the flux by forming magnetic circuits among the magnetic head, the magnetic recording layer, and the soft magnetic layer during magnetic recording.
However, there is a problem that the formation of the soft magnetic layer disrupts the orientation and the smoothness of the non-magnetic under layer and affects the perpendicular orientation of the magnetic recording layer. Regarding this problem, Patent Document 4 (Japanese Unexamined Patent Application Publication No. 2005-044464) discloses a configuration composed of a first non-magnetic under layer being amorphous and made of a metal element forming an fcc structure (face-centered cubic crystal structure) by itself alone, a second non-magnetic under layer having an hcp structure, and a perpendicular magnetic recording layer (magnetic recording layer) that are formed so as to be in contact with one another. According to Patent Document 3, by the above-mentioned configuration, the perpendicular orientation of the perpendicular magnetic recording layer can be strictly controlled.    [Patent Document 1] JP-A-2003-77122    [Patent Document 2] JP-A-2002-74648    [Patent Document 3] JP-A-2002-74648    [Patent Document 4] JP-A-2005-044464